This invention relates to an optoelectronic switch which switches an electrical signal by using light as a medium or carrier, and more particularly an optoelectronic switch utilizing a photodiode.
When compared with other types of switching elements, the optoelectronic switch using light as a carrier is characterized by high isolation corresponding to a large ON-OFF power ratio (.about.80 dB), broad transmission bandwidth (.about.3 GHz), the ability to switch analog and digital signals, and small cross-talk when used in switching arrays.
Although optoelectronic switches of various designs have been constructed, they have not yet been developed to a stage where electrical signals ranging from direct current to alternating current in the microwave frequency band can be switched at high speeds.
The following characteristics are required of optoelectronic switches utilized to switch telephone lines, or to distribute television signals, or to switch microwave lines and digital signal lines;
(1) The loss of the signal in the ON state should be as small as possible.
(2) The leakage of the signal in the OFF state should be as small as possible.
(3) High speed switching should be possible.
(4) The power consumption in both the ON and OFF states should be small.
(5) The cross-talk (leakage of signal) between signal transmission lines should be small.
Optoelectronic switches already developed can satisfy some of the requirements described above but cannot satisfy all of the requirements.
A typical optoelectronic switch was disclosed by E. H. Hara, one of the inventors of this application, and R. I. MacDonald in a paper titled "Optoelectronic Broadband Switching Array" in Electronic Letters, Vol. 14, pages 502 and 503, August 1978.
The optoelectronic switch disclosed in this paper comprises a combination of an electrical-to-optical converting element such as a light emitting diode and an optical-electrical converting element such as a p-i-n photodiode.
As is well known in the art, with this type of optoelectronic switch, the degree of optical-to-electrical conversion efficiency and impedance depends on the magnitude and polarity of the bias voltage applied to the photodiode. More particularly, with a forward bias, the conversion efficiency and the impedance of the diode is small, whereas when a reverse bias voltage is applied, the conversion efficiency is enhanced.
Usually, one terminal of the optical-to-electrical converting element is grounded via a resistor and the other terminal is selectively connected to two bias sources of opposite polarities through a transfer switch which may be of mechanical design or of semiconductor design. To render ON the optoelectronic switch, the transfer switch is thrown to a bias voltage which applies a reverse bias voltage to the optical-to-electrical converting element, and to render OFF the switch, the transfer switch is thrown to a bias source which applies forward bias to the converting element. By operating the transfer switch in a manner described above, an output corresponding to the incident light signal provided by the electrical-to-optical converting element is produced at a juncture between the converting element and the resistor in response.
It was found that the switching of the signal by switching the polarity of the bias voltage applied to the optical-to-electrical converting element produces the following problems.
(1) To render OFF the optoelectronic switch by applying the forward bias to the optical-to-electrical converting element, it is necessary to pass a current of the order of several milliampares to several tens of milliampares. For this reason, electric power consumption of the converting element and the resistor amounts to several milliwatts to several tens of milliwatts so that a comparatively large capacity electrical power source is required to operate the optical-to-electrical converting element. In addition, where a plurality of optoelectronic switches are used or where they are assembled into an integrated circuit, a comparatively large quantity of heat is generated and this is not desirable for the converting elements and associated electronic circuitry, because the higher temperature resulting from the heat degrades their performance and shortens their useful life time.
(2) Since it is necessary to switch the polarity of the bias source, not only does the circuit design become complicated but also it is necessary to use separate bias sources having different polarities and this is a disadvantage when design, final product size, and economy of manufacturing are considered.
(3) Since the impedance of the converting element varies greatly depending upon the polarity of the bias voltage (i.e., low in the forward bias condition, high in the reverse bias condition), impedance matching cannot be maintained at the output of the switch in the OFF state.
(4) Since the OFF state of the optoelectronic switch is produced partly by shunting of the load resistor by the junction capacitance of the photodiode in the forward bias, the isolation ratio is frequency dependent and at low frequencies the isolation ratio decreases.
(5) Where a p-n or p-i-n junction type photo-diode is used as an optical-to-electrical converting element, as is well known in the art, when the bias voltage is transferred from forward to reverse direction, the switching speed is limited by the charge storage effect at the junction. In such photodiodes, the switching speed ranges from hundreds of nano seconds to several microseconds, meaning that such photodiodes cannot be used for applications requiring high speed switching. As a solution, for the purpose of decreasing the charge storage effect, it has been proposed to diffuse gold into a silicon photodiode. This method, however, not only makes it difficult to manufacture the photodiode but also decreases its optical-to-electrical conversion efficiency. In addition, this method cannot obviate the above described disadvantages (1), (2) and (3).
Because of these disadvantages, the field of application of the prior art optoelectronic switches has been limited to circuit switching where switching speed is not a point of concern.